In recent years, the data writing method known as so-called “shingled write” has been developed as a technique achieving a high storage capacity, in disk storage apparatuses a representative example of which is the hard disk drive (HDD).
The shingled write method is a data writing method, in which data is written on one track and partly on the immediately adjacent track, in the radial direction of the disk. In the shingled write method, track group (band) composed of a plurality of tracks is defined as write unit. That is, data is rewritten in units of bands, in the shingled write method.
In any HDD, the magnetic recording intensity at each track may decrease because of the leakage of magnetic field from the adjacent tracks. This decrease in the magnetic recording intensity is called adjacent track interference (ATI). In the shingled write method, if data is written in a band, the magnetic recording intensity decreases at an outer or inner track of a track group adjacent to the band in which the data is written. To prevent the magnetic recording intensity from decreasing in the HDD using the shingled write method, a refresh process, rewriting the same data at the band, is performed.
In the HDD using the shingled write method, the number of times data has been written at the band adjacent to a specific band is counted, and the refresh process is performed when the count exceeds a threshold value. In the refresh process, the data is read from a band, temporarily saved in a memory and then written back to the band. This lowers the operating efficiency of the HDD. It is desired that the frequency of the refresh process should be minimized.